Field observations carried out in southern South Australia indicate that rock flakes 1–15 mm thick occur on some cavernous rock surfaces in granites and gneisses. Measurements of temperature conditions within caverns indicate that rapid temperature changes are unlikely to cause flaking, and only minor differences in relative humidity were recorded between the air within a cavern and the air outside. Hydration of some minerals points to the importance of moisture, which in the absence of extreme temperature variations, can evenly penetrate relatively unfractured rock and cause swelling of altered minerals. Fracture planes eventually develop along the boundary of innermost moisture penetration. Most noncaverned rock surfaces experience wide variations in temperature and moisture conditions, causing randomly oriented fissures to develop concurrently with a reduction in rock cohesion. On these surfaces, weathering proceeds principally by granular disintegration.